In the U.S., skin cancer is becoming as frequent as all other cancers combined. These tumors provide a rare oppportunity to study early events in carcinogenesis because the carcinogen is known, the lesions are observable, and some cancer types progress through defined stages. In the previous period we explained many aspects of tumor initiation, culminating in finding that UVB photons mutating the p53 tumor suppressor gene each lead to a separate precancerous lesion (Nature 372:773, 1994). We also reported that the p53 gene is part of a novel 'cellular proofreading' mechanism that causes UV-damaged keratinocytes to die by apoptosis (programmed cell death), removing precancerous cells from the skin. These results raise the possibility that cellular proofreading is a key determinant of the extent of clonal expansion during radiation carcinogenesis. We wish to address 3 salient questions: i) Is cellular proofreading important for human skin cancer? ii) Does the particular p53 mutation in a tumor determine the balance between cellular proofreading and the G1 arrest/DNA repair function of p53? iii) Is the cellular proofreading mechanism manipulatable? Our hypotheses are: i) Loss of cellular proolreading is responsible for preneoplastic lesions in human skin. ii) Different mutant p53 alleles found in human skin cancers and precancers have different phenotypes for transcriptional-activation of apoptosis- related versus G1 arrest-related genes, and for apoptosis. iii) Transgenic mice with increased skin cancer frequencies, and mice treated with tumor promoters or chemopreventative agents, will often have an altered cellular proofreading mechanism. The last hypothesis also implies two routes of tumor promotion: a) mutations in genes that result in rare apoptosis- resistant keratinocytes, whose clonal expansion will be favored by future sunlight-damage; b) Interference with cellular proofreading in all keratinocytes, impeding removal of precancerous cells. Specifically, we will: 1. Determine whether precancerous lesions in human skin have lost competence for UV-induced apoptosis. 2. Determine whether different mutant p53 alleles in human skin cancers and precancers have different phenotypes for transcriptional-activation of apoptosis-related versus G1 arrest-related genes, and for apoptosis in cultured keratinocytes. 3. Determine which tumor promoters or chemopreventative agents act by influencing UV-induced cellular proofreading. 4. Determine which genes that affect skin cancer in transgenic mice act by influencing UV-induced cellular proofreading. These experiments test the importance of cellular proofreading in preventing skin cancer. In addition, the pharmacologic agents and genes found to be effective, and the in vitro apoptosis system, will form the basis for continuing this work to elucidate molecular pathways. If cellular proofreading is manipulatable, it should be possible to control tumor regression.